This invention relates to a novel method for slurry coating the surface of the viewing window of a faceplate panel having a peripheral sidewall. The novel method may be applied particularly to coating a layer of phosphor particles upon the inner surface of the viewing window of a faceplate panel, which panel is subsequently used to make a cathode-ray tube; for example, a color-television picture tube.
In one method for making a luminescent viewing screen for a color-television picture tube, a quantity of slurry in excess of what is required to make the screen is dispensed on and spread over the inner surface of the viewing window of a faceplate panel. The panel comprises a central glass viewing window and an integral peripheral sidewall or flange around the window, the distal end of which is a sealing land. The slurry comprises a mixture of phosphor particles, a binder such as polyvinyl alcohol, a photosensitizer for the binder such as sodium or ammonium dichromate, and a liquid vehicle such as water. The slurry is spread by rotating and tilting the panel so that the slurry spreads evenly over the window surface. At the time of dispensing, the temperature of the slurry is in the range of 18.degree. to 30.degree. C., while the temperature of the panel is in the range of 25.degree. to 50.degree. C. The excess slurry is then removed, as by rapidly spinning the panel to move the excess slurry by centrifugal force up the sidewall and over the sealing land and to sling the excess slurry from the panel.
By another method for removing the excess slurry, the panel is slowly rotated and then tilted to a high angle to move the slurry across the sidewall to the sealing land and to dump the excess slurry from the panel. By either method for removing the excess slurry, a variable area of the sidewall is coated with the slurry. The boundary of this area is not predictable, but usually falls close to the viewing window at some places and crosses the sealing land at other places. Then, the sealing land and the adjacent sidewall are cleaned, preferably ultrasonically, to remove residual particulate matter.
Because some drying occurs during the time period between the step of removing the excess slurry and the step of cleaning the sidewall, a bead comprising particulate matter of the slurry forms at the above-mentioned boundary, which bead is more difficult to remove than the adjacent coating. Also, a portion of the bead frequently is located too close to the window to prevent it from being removed economically. By the novel method, the bead is formed close to or at the sealing land where it can be removed economically by ultrasonic or mechanical means. The foregoing steps for coating the panel are usually conducted on automatic or semiautomatic machines in which a series of panels pass single file through a sequence of stations in which the steps are carried out. The novel method may be carried out on such machines.